Dispensing with large-volume electric energy storage equipment and low pollution to the harmonic of the power grid, the Matrix Converter (MC) is an AC-AC electric power converter having a compact structure and a high power density. Recently, with the continuous improvement of the commutation technology, system stability, control modulation policies and other aspects, an MC-fed motor speed control system has been applied in many industrial fields such as elevator traction, wind power generation and machine manufacture.
The Direct Torque Control (DTC) was proposed in 1986 for controlling a Voltage Source Inverse (VSI)-fed induction motor speed control system (VSI-DTC) in early. Due to its advantages of simple structure, independence from motor parameters, no need for rotational coordinate transformation and the like, DTC has attracted much attention of scholars. As the MC control modulation technology gradually becomes mature, scholars in other countries than China have proposed a novel DTC in 2001 and applied the novel DTC in an MC-fed induction motor speed control system (MC-DTC). This method may not only directly control an electromagnetic torque and a stator flux on the motor side, but also control an input power factor angle on the grid side. However, as both the MC-DTC and the VSI-DTC employ a control structure using a hysteresis comparator and a voltage vector switching table and only one voltage vector is used within each control cycle, the motor system has two main defects, i.e., too large torque ripple and inconstant switching frequency. To solve those defects, many suitable improved DTCs for VSI have been proposed continuously. Subsequently, scholars worldwide optimize the improved DTC algorithms and utilize the improved DTC algorithms in the MC-fed motor speed control systems. The improvement of algorithm may be classified as below.
1. Multi-stage hysteresis and multi-vector subdivision are employed. For VSI, as it has only 6 effective voltage vectors constant in amplitude and direction, it is required to generate 56 virtual voltage vectors with unequal amplitude by discrete spatial vector modulation and subdivide and select the virtual vectors by a five-stage torque hysteresis comparator, thereby realizing the purpose of inhibiting torque ripples. For MC, as it is characterized by multiple vectors, i.e., 18 amplitude variable vectors distributed in 6 directions, the voltage vectors may be subdivided into large and small vectors according to the amplitude, and the voltage vectors are selected by a five-stage torque hysteresis comparator, thereby realizing the inhibition of the torque ripples. Studies have shown that, although such a method maintains the DTCs' advantages of simple structure, no need for motor parameters and rotational coordinate transformation and the like and has good inhibition effect on torque ripples, it has a defect of inconstant switching frequency.
2. The voltage vector switching table (DTC-SVM) is replaced by an SVM. In such a method, by taking torque and flux offsets as input, adopting a PI controller, a deadbeat controller, a sliding mode controller, a prediction controller and the like to acquire a reference voltage value of a motor stator, and utilizing the SVM to acquire an actual voltage vector according to the reference value. Some scholars apply this method to MC, where the MC is equivalent on a virtual rectifier side and a virtual inverter side, and the SVM is applied in part or all of the virtual rectifier and inverter sides to acquire an optimal input current or output voltage. As the SVM may generate continuously rotational voltage vectors in a complex plane, this method may accurately control the motor torque and flux. However, as the control structure thereof is more complicated than a conventional DTC, rotational coordinate transformation and large amount of calculation are often required.
3. Mark-to-space ratio optimization calculation is employed. In such a method, a conventional DTC switching table is employed to select a voltage vector and the mark-to-space ratio of this voltage vector is calculated by a torque optimization formula so that torque offset within one cycle is minimized. Such a method obviates the need for rotational coordinate transformation, may inhibit torque ripples well and has a constant switching frequency. However, a majority of optimization algorithms are complicated and highly depend on motor parameters.
Those three improved algorithms realize the purpose of inhibiting motor torque ripples at the cost of sacrificing some inherent advantages of the direct torque control. These algorithms, either they are complicated in calculation or depend on motor parameters or need for rotational coordinate transformation, can not improve their disadvantages on the basis of strengthening their advantages in control.